What you get is not a function handle but an heterogenous array of SimBiology objects. This array contains a mix of different component types like species, parameters or potentially compartments.
If this is what you want, you can get their names with
names = get(VariableSpecies, 'Name')
But if you only wanted species (not parameters nor compartments), you can get them like this:
speciesObj = sbioselect(modelObj, 'Type', 'Species','Where','Constant','==',false)
names = get(speciesObj, 'Name')